Data is often transmitted from a business machine at one location over telephone lines to a business machine at a remote location. The telephone lines characteristically introduce error to the signal being transmitted. To overcome this, the receiving station must be adaptive to the characteristics of the telephone lines. This adaptation or adjusting function is typically performed by a modem, and the modem field is now highly developed.
When it is desired to transmit data from a transmitting station to a receiving station, the transmitting station generally transmits a start-up sequence or preamble of known characteristics. In response to this preamble, the receiver at the receiving station must first confirm that the preamble is a signal rather than noise and then make the necessary adjustments so that the receiving station can properly receive and process the data signal.
It is desirable to reduce the learning time of the receiver to a minimum so that the modem can process more data in a given length of time. In a multi-point communication network comprising a master station and numerous slave stations, any reduction in the learn time is multiplied by the number of stations in the network for each poll of the network. Thus, a fast learn modem enables more efficient use of the telephone network.
One fast learn modem is disclosed in Motley et al U.S. Pat. No. 3,962,637. This modem utilizes a preamble which includes an unmodulated carrier portion, a modulated carrier portion and two impulses separated from each other and from the adjacent portions of the preamble by a null or zero energy portion. Although the modem of this patent functions very well and provides fast adaptation to the communication channel, for some applications, even faster learning is desired.